Fluxes in calcium ion concentration and in cyclic nucleotide concentrations are believed to be involved in responses of human blood platelets to various stimuli. This project is designed to investigate the structure and function of a low molecular weight calcium-binding protein (CaBP) which I have purified from platelets. Structural studies include determination of the number of calcium binding sites by equilibrium dialysis and fluormetry, competition for these sites by other ions and various drugs, effect of bound calcium on tertiary structure as measured by spectrophotometric titration of tyrosine residues, and peptide mapping after digestion with trypsin in the presence and absence of calcium. Functional studies include measurement of the activation, in the presence of calcium and CaBP, of some forms of cyclic nucleotide phosphodiesterase, the possible involvement of an inhibitory protein interfering in phosphodiesterase activation, and the effect of certain drugs on the activation. Other studies include measurement of myosin light chain kinase activity in the presence of calcium and CaBP, and measurement of adenyl cyclease activation at very low calcium levels in the presence of CaBP.